Congestion in cellular radio networks is a continuing concern as both the number of users and especially the volume of data being handled continues to increase. In a typical cell there will be a number of user equipments (UEs) in a connected state with the cell and a number of other UEs in an idle state. Only those in the connected state can send and receive user data, those in the idle state listen at prescribed times in case there is an incoming call to them, and they also measure and report on neighbor cells so they can be redirected or handed over efficiently as they move across the cell.
Different radio access technologies (RATs) use different terms for the idle and the connected states. For example, the Universal Terrestrial Radio Access (UTRA) has several different states beyond the actual IDLE state which may be considered as idle or at least semi-idle, including the cell forward access channel (CELL-FACH) state (semi-idle), the cell paging channel state (CELL-PCH) state (semi-idle) and the UTRA registration area paging channel state (URA-PCH). The common feature of these UTRA states is that the UE is not allocated a dedicated channel (DCH) for user traffic. The Evolved Universal Terrestrial Radio Access (E-UTRA) system, sometimes referred to as long term evolution (LTE), terms the UE as being in either an idle mode or a radio resource control (RRC) connected mode. Unless otherwise indicated herein, the terms idle (or semi-idle) state and connected state are used in a generic fashion and not specific to any RAT. When a UE is camped on a cell this also indicates it is in an idle or semi-idle state.
When a UE in an idle/semi-idle state attempts to establish a connection with a radio network cell such as to place a call or access email or some social network, the radio network currently has no means by which to re-direct the UE to instead connect to an alternate frequency. Current radio network procedures require the UE's connection request to be either rejected outright or accepted. Network operators prefer not to reject connection requests (a RRC Connection Reject message in E-UTRAN), but if the cell is congested that leaves the network only the option of allowing the UE's requested connection and handing it over soon after to another frequency. If congestion is too severe this may not be possible, but if it is the process of accepting the UE's connection request increases cell congestion on the original congested frequency until the redirection can take place. What is needed is a more effective procedure for radio networks to carry out load balancing with new connection requests.
One technique to handle such overloads was proposed for the Universal Mobile Telecommunication System (UMTS, also UTRA) in document R2-105129 which is a change request for 3GPP TS 25.331 (3GPP TSG RAN WG2 Meeting #71; Madrid, Spain; 23-27 Aug. 2010). In this approach the UE sends capability flags in its RRC Connection Request message to indicate which frequency bands the requesting UE supports so the network knows the UE's capability. Knowing the UE capability is not enough, because the network needs to know whether the UE sees this neighbor frequency with sufficient strength and/or quality to connect otherwise the UE might not be able to establish the connection. While the Measurement Report message for inter-RAT LTE measured results can be reported by a UE in the CELL-FACH state, it is not clear how these results can be signalled in the LTE idle mode or in the UTRA CELL-PCH/URA-PCH states, so at best this appears a partial solution only.
For the E-UTRA system, the inventors are aware of two relevant proposals. Document R2-122553 entitled Network Controlled CELL_FACH Mobility (3GPP TSG-RAN WG2 Meeting #77bis; Prague, Czech Republic; 21-25 May 2012) includes E-UTRA measured results in an uplink RRC message so that the radio network can redirect the UE to the measured E-UTRA frequency. Document R2-122609 notes disadvantages with a prior de-prioritisation request signalled to the UE via a RRC Connection Reject message and proposes instead to signal re-direction information via the RRC Connection Reject message. Some reservations have been expressed in that the network will not at that time know the UE's capabilities and so the re-direction might send the UE to a neighbor frequency it does not support. This does not appear solvable by including the UE's measured results of the frequencies of RATs in the UE's RRC Connection Request message, since there appears insufficient space in this uplink control channel message to do so. The similar constraint is seen to prevent the UE reporting such results in its Cell Update message while in the CELL-PCH or URA-PC state.